This invention relates to thin concrete or stucco wall panels. In particular this invention relates to means for securing stucco wall panels to wood frames to improve the shear resistance to cyclic loading such as from earthquakes or wind.
In building and structural design, structural elements need to be incorporated in the design to adequately withstand rack shear loads imposed by earthquakes or high winds. Common methods to achieve this shear strength are through the use of moment resistant frames, cross bracing, or through shear resistant structural diaphragms. Wood frame shear walls are a primary lateral force-resisting element in wood frame structures. Their behavior is often considered analogous to a deep cantilever beam with the end-framing members acting as xe2x80x9cflangesxe2x80x9d or xe2x80x9cchordsxe2x80x9d to resist overturning moment forces and the panel acting as the xe2x80x9cwebxe2x80x9d to resist shear. This analogy is generally considered appropriate for wind and seismic design. The structural elements of the building wall of interest are described by properties such as strength, stiffness, and ductility. Common diaphragm materials would be plywood sheathing, oriented strand board (OSB), or gypsum wall board (GWB).
In certain areas of North America, such as the Western States of the U.S., another material that is commonly used to provide shear strength is stucco cladding. Stucco is a Portland cement plaster that is reinforced with either wire lath, or expanded metal lath.
The stucco is normally xe2x85x9e inch thick and achieves compressive strengths of 1,500 to 2,800 psi.
The lath serves several functions. First, it provides reinforcement for the stucco to provide tensile strength, similar to structural concrete design. Second, it provides the means for attachment of the stucco cladding to the building framework. The lath is attached in a prescribed manner. Usually fasteners are applied every 6 inches vertically on studs and every 6 inches horizontally along the top and bottom plates for each storey. In the case of wood framing, the fasteners are often roofing nails but they may be staples.
With the use of such stucco wall cladding, engineers have been allowed to use a shear value of 180 lbs. per foot to resist rack shear forces in their designs. Following the Northridge Calif. earthquake of Jan. 17, 1994, it was found that some buildings constructed with stucco shear walls suffered premature failures. Subsequently, some building officials have lowered the shear rating for stucco to 90 lbs. per foot in specific seismic areas.
Failure of the stucco cladding was primarily caused by failure of the attachment of the reinforcing lath to the framing. The normal method of attaching the lath is to drive a nail alongside a wire and hold the wire under one side of the head of the nail. Where possible, the nails are driven at the intersections of the wire lath, but the wires are still only held under one side of the nail head. For unidirectional loading, such as wind forces, the fastening is adequate. However, during reversing cyclic loading such as an earthquake, the fastening is inadequate. During an earthquake of high intensity, there is relative movement between the stucco cladding and the building framing. With displacement in the first direction a number of fasteners will disengage from the lath. With displacement in the other direction, more fasteners will disengage. Over a few cycles, the majority of fasteners will be disengaged.
Since the wire sizes in wire laths used in the Northridge Calif. earthquake were relatively small, usually 0.050 to 0.062 inches in diameter, they had limited strength. Under cyclic loading where the nails did not disengage from the wire, the loading broke the wire. On some buildings, construction staples had been used to attach the lath. The performance of staples was worse than the nails, since it was found that each leg sheared with the reversing cyclic loads. Accordingly, there is a need for an improved method for transferring rack shear forces to a building frame.
Wood framing for a wall without sheathing or diagonal bracing is essentially a low strength structure that deforms readily under shear loads. Shear walls have traditionally used sheathing, such as plywood, that is very stiff under shear loads to reinforce the wood framing. The sheathing is fastened to the wood framing by nails using a nailing pattern that meets engineering requirements. Under in-plane shear loads, the nail fasteners are the ductile elements that deform and provides the inelastic behaviour that is essential for energy dissipation in cyclic loading.
Stucco wall panels can provide effective reinforcement for wood framed shear walls if a suitable method is used to attach the panels to the wood frame. Nails, such as used to attach plywood sheathing to a wood frame, are not suitable for stucco. Nails will pull out of the stucco by local brittle fracture mechanism under tension loading. Nails will crack the stucco when the local yield stress is exceeded under shear loading.
The problem of providing attachment between a shear-resisting element and a structure under in-plane shear loading has been addressed by many building structure designers. In 1999, Utzman was granted U.S. Pat. No. 5,870,870 for metal shear panel fasteners to fasten shear resistant diaphragms to framing elements in the construction of framed building. Ulzman claims a special fastener similar to a xe2x80x9cduplexxe2x80x9d nail for attaching the diaphragms to the wood building frame. The application was to attach metal-faced gypsum wallboard to wood frame construction for shear walls. Although Utzman claims that his invention provides good shear resistance, the system is not suitable for fabricated-in-place stucco wall finishes.
In 1996, Irimies was granted U.S. Pat. No. 5,493,833 for a welding stud used in the construction industry for providing a shear connection between cast-in-place concrete and structural steel. Irimies provided the optimum dimensions for an array, or group, of studs to develop full shear strength when embedded in concrete. The welding stud has applications only in cast-in-place concrete structures that are attached to steel structural members to which the studs can be welded. The objective of that shear connection is to create a joint between the concrete and the structural steel that is stiff and strong. The connection does not, however, provide ductility and is not suitable for wood frame construction.
In 1995, Ruiz et. al. was granted U.S. Pat. No. 5,414,972 for a reinforced structural member to be used in the construction industry for providing a shear connection between a concrete building panel and a reinforcing member that will be fastened to a building structural member. The main application is to attach pre-fabricated building panels to a steel building framework. Ruiz shows a reinforcing member having a multiplicity of projections that embed in concrete to provide full shear strength. As in the Irimies invention, this shear connection is not capable of providing any ductility.
In 1989, Grimaud was granted U.S. Pat. No. 4,841,703 for a connection system to create a composite beam structure from a concrete slab cast over wooden floor beams. The connectors are tubes inserted into the concrete slab and the wood floor beams along the common interface to provide sufficient material to carry the shear loads. The object of the invention is to create an integral structure with no relative motion between components. Grimaud made reference to another connection system that uses vertical nails that are partially engaged in the faces of the wood floor beams and embedded into the concrete during casting. [Godycki et. al. xe2x80x9cVerbunddecke aus Holzrippen unde Betonplattexe2x80x9d, Bauingenieur 59 (1984), 477-483, Springer-Verlag, Germany]. Such connectors suffer from limited shear strength and either deform readily or break at the interface, and hence are not suitable for attaching thin vertical panels of stucco cladding to wood frame walls.
The object of the present invention is to provide a thin concrete or stucco wall construction system that provides improved shear resistance and ductility to in-plane cyclic shear loading.
This invention has a primary function of transferring shear loads between the stucco wall panels and the wood building frame, especially during cyclic shear loading of a wall structure. An important secondary function of the shear fastener is to permit relative motion between the essentially inelastic stucco wall panels and the relatively flexible wood building frame. The design of the shear fastener provides energy dissipation due to inelastic action during the cyclic motion. The strength and stiffness of the wall during shear loading are important parameters. The shear load per unit of lateral displacement (drift) is the stiffness of the wall structure. For low shear loads, the wall structure is often elastic wherein the lateral displacement reduces to zero when the load is removed. At higher loads, the lateral displacement is not recovered when the load is removed indicating inelastic behaviour of the wall system. Inelastic behaviour means that some elements of the wall are permanently deformed by the shear loads.
An element of the invention is a connector similar to a shear stud known in the construction industry for bonding concrete to steel floor decks, structural beams, and similar elements. In this invention, the connector transfers shear force between the wood building frame construction and the stucco wall panels, in particular seismic shear force due to earthquake motions of the building structure.
A shear stud connector comprises metal studs, each having at one end a head or section with a larger diameter than the body of the metal stud, also having a second section of larger diameter part way along the metal stud. The concrete is applied to fully encase the body of the metal stud in the region between the two increased sections. The embedded shear capacity of a metal stud depends on a number of factors including: (a) the properties of the concrete such as weight, compressive strength, and modulus of elasticity; (b) the properties of the metal stud such as the shank diameter, the head size, and the yield strength of the metal stud material; and (c) the geometric properties of the shear connector such as the metal stud spacing and grouping.
When the shear fasteners of this invention are used in an approved method, they are keyed rigidly into the stucco wall panels and inserted into the wood building frame. The fasteners then carry both static and dynamic tension and shear loads consisting of the dead weight of the stucco wall panels and wind and seismic loading on the building.
The basic concept is that stucco is a stiff and strong material that, if properly connected to the wood frame structure, is capable of resisting significant in-line shear forces (similar to or greater than plywood shear walls). Currently, the seismic shear connection between stucco and the wood frame construction relies on the nailed connection of the wire lath to the wood studs, which is inadequate for cyclic loading. With the addition of the connector described in this invention to the stucco clad building wall, the magnitude of the shear that needs to be transferred between the wood frame, wire nails, reinforcing wire, and stucco panels is greatly reduced. The main purpose of the traditional connection is to tie the stucco and studs together in order to prevent buckling of the thin stucco, and to help support the dead weight of the stucco.
The invention has been developed so that, in a stucco shear wall, the inelastic action occurs primarily in the fasteners between the wood building frame and the stucco wall panels. Further, the invention has been developed so that the inelastic action in the fasteners is confined to the interface region between the stucco and the wood building frame including permanent deformation of the wood frame at the insertion regions of the fasteners. Ultimately, the stucco wall panel will develop cracks for large seismic motion.
The fastener element of this invention has been developed to extend an optimum distance into the stucco to provide the required strength, but at the same time maintaining adequate cover of the stucco to protect the fastener from corrosion. The fastener is preferably made of corrosion resistant materials or plated with a corrosion resistant material such as zinc to improve its function.
In one embodiment, the fastener consists of an elongated portion that is driven into a wood frame wall, and an elongated second portion that is completely embedded in the stucco wall panel. In a further embodiment, the fastener consists of two parts; the first being a shaft with a headed end and a pointed end that is driven into the wood frame wall, and the second part being an annular body that is embedded rigidly into the cement stucco wall. The first part of the fastener passes through the annular body of the second part to lock the two components together. The external shape of the annular body is developed to provide adequate anchorage against combined bending and pullout from the stucco, which sets around it. The internal hole in the annular body is sized to fit the shaft of the first part of the fastener.
The fastener of this invention has been developed to provide adequate pullout resistance from the wood building frame, while at the same time providing appropriate bending stiffness for the energy absorption function of fastener. The grade of steel used to form the shear lock provides the required strength, while the type of steel used in the device provides adequate low-cycle fatigue capacity. Typically, the material for the fastener could be similar to low carbon 1008 steel that has been work hardened to provide a yield strength of 70,000 psi although many other variations are possible.
When the fastener system is applied in the proper locations described below, and is combined with other important properties of reinforced stucco, the resulting system may be characterized as an xe2x80x9cengineered stuccoxe2x80x9d system that is capable of resisting very significant seismic shear forces.
The fasteners should be provided along all horizontal edges where shear is to be transferred between the wood frame construction and the stucco wall panels. These are primarily along the bottom and top plates of shear walls at each floor level. In addition, for shear wall panels that have height-to-width ratios greater than 0.5, the connectors must be provided along vertical edges of the wall panels, in addition to the horizontal edges.
In one aspect the invention is the method of applying stucco to a wall having wood frame members wherein the stucco is applied to a lath, the method comprising fastening the stucco directly to the wood frame members separate from the lath. The direct fastening may be through a building sheathing membrane.
In another aspect of the invention, the method comprises securing stucco lath to the wood frame members or building wall sheathing by means of first fasteners, securing a plurality of second fasteners to the wood frame members and applying stucco over the lath and the second fasteners whereby to embed a portion of the second fasteners in the stucco.
In another aspect, the invention is a stucco wall construction comprising wood frame members, lath secured to the wood frame members, a panel formed by the stucco on the lath, and means separate from the lath to fasten the stucco directly to the wood frame members.
The means for fastening the stucco directly to the wood frame may comprise fasteners having a first elongated portion that is driven into the wood frame members, and a second elongated portion that is completely embedded in the stucco panel.
In an other of its aspects, the invention is the method of applying stucco on the wood frame of a building wall comprising the steps of: securing stucco lath to the wood frame by means of a plurality of first fasteners, embedding in the wood frame a plurality of second fasteners, the second fasteners comprising an elongated first portion embedded in the wood frame and an elongated second portion not embedded in the wood frame, the second portion including a head spaced from the surface of the wood frame, and, applying stucco to the stucco lath so as to substantially embed the heads of the second fasteners in the stucco.
In yet another of its aspects, the invention is a stucco wall construction comprising a wood frame, stucco lath secured to the wood frame by a plurality of first fasteners, a plurality of second fasteners embedded in the wood, the second fasteners comprising an elongated first portion embedded in the wood frame and an elongated second portion not embedded in the wood frame, the second portion including a head spaced from the surface of the wood frame and stucco applied to the stucco lath so as to substantially embed the heads of the second fasteners in the stucco.
In yet another of its aspects, the invention is a fastener assembly for use in a stucco wall construction comprising a first element having an elongated portion and a head portion, the elongated portion having a substantially uniform cross section along its length, and a second element having an aperture for receiving the first element therethrough, whereby to maintain the head of the first element in a predetermined spaced relationship to a surface in which the first element is embedded.
In another of its aspects, the invention is a method of applying to stucco to a wall having wood frame members wherein the stucco is applied to a lath or a wire mesh, comprising fastening the lath or wire mesh to the wood frame members by means of first fasteners, securing a plurality of the fastener assemblies by driving the elongated portion of the first element of the assembly through an aperture in a second element of the assembly and into the wood frame member, and applying stucco to substantially embed the second element in the stucco.
In another aspect, the invention is a sleeve element for use in conjunction with a fastener for a stucco wall construction comprising a first end and a second end defining a longitudinal axis, an axial passageway extending along the longitudinal axis, and an outer surface between the first and second ends, the outer surface having a varying cross section along its length.
In another aspect, the invention is a stucco wall construction comprising a wood frame, stucco lath secured to the wood frame by a plurality of first fasteners, a plurality of second fasteners embedded in the wood, the second fasteners comprising a fastener assembly having a first element having an elongated portion and a head portion, the elongated portion having a substantially uniform cross section along its length, and a second element having a passageway for receiving the first element therethrough, whereby to maintain the head of the first element in a predetermined spaced relationship to a surface in which the first element is embedded.
In yet a further of its aspects, the invention is a fastener assembly for use in a stucco wall construction comprising a first element having an elongated portion and a head portion, the elongated portion having a substantially uniform cross-section along its length, and a second element having a passageway for receiving the first element therethrough, whereby to maintain the head of the first element in a predetermined spaced relationship to a surface in which the first element is embedded, the second element having an upper end and a lower end, and wherein the second element encloses the head of the first element in the upper end so as to restrain it from relative upward axial movement.
Other aspects of the invention will be appreciated by reference to the description of the preferred and alternative embodiments thereof and to the claims wherein the various additional aspects of the invention are defined.